In 1886 he renounced his promising ecclesiastical career, and entered the BenedictineAbbey of Maredsous in Belgium, taking the name Columba. Nearly 30 at the time of his noviate, and forced to learn a new language at the same time, he persevered and made his solemn profession on 10 February1891.

While the walls stood, World War I effectively destroyed his abbey. Germanlay-brothers, who had been there for years, were expelled from Belgium. Columba sent young monks to study in Ireland; they would be safer there, but the remaining brothers, including Columba, had to shoulder more work. After the war, a group of the brothers were sent to the Monastery of the Dormitian in Jerusalem, which had been left nearly deserted when the British expelled Germanmonks. Amidst it all, Columba continued to preachretreats, guide the lost, and write.

O Lord, our God, You have given Your servant, Columba, to understand so well that by our Baptism we are Your adopted children and brothers and sisters of Jesus Christ: as a good Father graciously hear the prayers which we address to You to obtain through his intercession this favor. Teach us by his example and doctrine how to live generously as true Christians with a childlike confidence and a simplicity abounding in love. Likewise, is such is Your pleasure, show clearly by miracles, how pleasing Your servant is to You, so that Holy Mother Church may be able to declare him a Saint. Through Jesus Christ our Lord. Amen. – prayer for the Canonization of Dom Columba Marmion